1. Field of the Invention
The present invention relates generally to electro-optical inspection systems, and more particularly to a method or algorithm for automated photomask inspection to detect defects on optical masks, reticles, and the like.
2. Description of the Related Art
Integrated circuits are made by photolithographic processes which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. A high production yield is contingent on having defect free masks and reticles. Since it is inevitable that defects will occur in the mask, these defects have to be found and repaired prior to using the mask.
Automated mask inspection systems have existed for several years. The earliest such system, the Bell Telephone Laboratories AMIS system (John Bruning et al., “An Automated Mask Inspection System—AMIS”, IEEE Transactions on Electron Devices, Vol. ED-22, No. 7 July 1971, pp 487 to 495), used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask using die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other. Other systems have been developed that teach die-to-database inspection, i.e. inspection of the reticle by comparison to the database from which the reticle was made.
As the complexity of integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater speed requirements, in terms of number of picture elements per second processed. The increased demands have given rise to improvements described in a number of subsequently issued patents, such as U.S. Pat. No. 4,247,203, entitled “Automatic Photomask Inspection System and Apparatus”, Levy et al., issued Jan. 27, 1981; U.S. Pat. No. 4,579,455, entitled “Photomask Inspection Apparatus and Method with Improved Defect Detection” Levy et al., issued Apr. 1, 1986; U.S. Pat. No. 4,633,504, entitled “Automatic Photomask Inspection System Having Image Enhancement Means”, Mark J. Wihl, issued Dec. 30, 1986; and U.S. Pat. No. 4,805,123, entitled “Automatic Photomask Inspection and Reticle Inspection Method and Apparatus Including Improved Defect Detector and Alignment Subsystem”, Specht et al., issued Feb. 14, 1989. Also of relevance is some prior art in the wafer inspection area, such as U.S. Pat. No. 4,644,172, entitled “Electronic Control of an Automatic Wafer Inspection System”, Sandland et al., issued Feb. 17, 1987.
Another force driving the development of improved inspection techniques is the emergence of phase shift mask technology. With this technology, it becomes possible to print finer line widths, down to 0.25 micrometers or less. This technology is described by Burn J. Lin, “Phase-Shifting and Other Challenges in Optical Mask Technology”, Proceedings of the 10th Annual Symposium on Microlithography, SPIE,—the International Society of Optical Engineering, Vol. 1496, pages 54 to 79.
The above improvements teach the automatic detection of defects on conventional optical masks and reticles. In all of these systems, conventional lighting is used and the images are captured by linear array sensors. These two system choices limit the signal-to-noise ratio and hence the speed of inspection.
Additionally, photomasks are used in the semiconductor manufacturing industry for the purpose of transferring photolithographic patterns onto a substrate such as silicon, gallium arsenide, or the like during the manufacture of integrated circuits. The photomask is typically composed of a polished transparent substrate, such as a fused quartz plate, on which a thin patterned opaque layer, consisting of figures, has been deposited on one surface. The patterned opaque layer is typically chromium with a thickness of 800 to 1200 angstroms. This layer may have a light anti-reflection coating deposited on one or both surfaces of the chromium. In order to produce functioning integrated circuits at a high yield rate, the photomasks must be free of defects. A defect is defined here as any unintended modification to the intended photolithographic pattern caused during the manufacture of the photomask or as a result of the use of the photomask. Defects can be due to a variety of circumstances, including but not limited to, a portion of the opaque layer being absent from an area of the photolithographic pattern where it is intended to be present, a portion of the opaque layer being present in an area of the photolithographic pattern where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or opaque layer. During the manufacture of photomasks, automated inspection of the photomask is performed in order to ensure freedom from the aforementioned defects.
There are, at present, three general methods for the inspection of patterned masks or reticles. One of those inspection methods is a die-to-die comparison which uses transmitted light to compare either two adjacent dies or a die to the CAD database of that die. These comparison-type inspection systems are quite expensive because they rely on pixel-by-pixel comparison of all the dies and, by necessity, rely on highly accurate methods of alignment between the two dies used at any one time for the comparison. Apart from their high costs, this method of inspection is also unable to detect particles on opaque parts of the reticle which have the tendency to subsequently migrate to parts that are transparent and then cause a defect on the wafer. One such die-to-die comparison method of inspection is described in U.S. Pat. Nos. 4,247,203 and 4,579,455, both by Levy et al.
The second method for inspecting patterned masks is restricted to locating particulate matter on the mask. It makes use of the fact that light scatters when it strikes a particle. Unfortunately, the edges of the pattern also cause scattering and for that reason these systems are unreliable for the detection of particles smaller than one micrometer. Such systems are described in a paper entitled “Automatic. Inspection of Contaminates on Reticles” by Masataka Shiba et al., SPIE Vol. 470 Optical Microlithography III, pages 233-240 (1984).
A third example of a system for performing photomask inspection is disclosed in U.S. Pat. No. 5,563,702 to David G. Emery, issued Oct. 8, 1996. The system disclosed therein acquires reflected images, in addition to transmitted images, to locate defects associated with contaminants, particles, films, or other unwanted materials. Since this system locates defects without reference or comparison to a description or image of the desired photomask pattern, it does not locate defects associated with photomask pattern errors, dislocations, or irregularities.
It has further been found to be advantageous to acquire both transmitted and reflected images for inspection of a photomask pattern with a die-to-die or die-to-database system. In particular, this approach has benefits for Embedded Phase Shift Mask (EPSM) inspection and Alternating Phase Shift Mask (APSM) inspection. Transmitted EPSM images often contain unfavorable optical characteristics due to partial coherence and interference induced by phase shifting. Some EPSM defects are simply either undetectable or indiscernible using existing die-to-die or die-to-database systems with transmitted images, but are nevertheless undesirable. Acquisition of both transmitted and reflected images for EPSM inspection has.
APSMs are typically designed with thickness variations in the glass or quartz which induce phase shift transitions between adjacent regions during photolithography. Phase defects can exist which are unwanted thickness variations created by phase etch process errors, and have similar optical image signatures during inspection. Hence APSM phase defects are difficult to distinguish from design phase features using the system shown in U.S. Pat. No. 5,563,702. Phase defects cannot be detected by this system without producing false defect readings on phase shift design features where no defects actually exist. However, the transmitted and reflected imaging capabilities and defect detection operators of this system can be useful to determine the presence of phase defects if all detected phase features are properly compared and contrasted to reference photomask image data, as in a die-to-die or die-to-database system.
Furthermore, the phase defect signal in brightfield transmitted light can be substantially less than that of a similarly sized chrome defect, thereby complicating the ability to inspect the mask. The phase defect's signal depends on a variety of factors, including the height or phase angle of the phase defect, the depth of the phase shifters, and inspection system optical parameters.
The use of reflected light in combination with transmitted light may improve detection of phase defects. The difficulty with using reflected light is managing image artifacts, such as the bright chrome halos resulting from the removal of the antireflective chrome layer during quartz etching of phase shifters. Bright chrome halos may have variable widths resulting from second write level registration tolerances with intra-plate variations. These variations are not observable when solely using transmitted light inspection techniques.
Thus, in general, phase feature signals captured with brightfield transmitted light may vary widely depending on mask and defect characteristics, and phase feature signals captured in reflected light may be stronger. On the other hand, use of reflected light can be problematic in the presence of image artifacts such as bright chrome halos. Therefore, die-to-die or die-to-database photomask inspection with transmitted and reflected light may benefit from signal-to-noise enhancements as well as an enhanced ability to discern phase shift features and phase defects.
Some references have suggested inspecting photomask substrates utilizing both transmitted and reflected light and have mentioned the possible use of both to classify defects. However, none of these references have provided details on a design using transmitted and reflected light to locate photomask pattern defects, and none have addressed the problems associated with the use of reflected light in the presence of bright chrome halos, or in the presence of particular anomalies when inspecting using brightfield transmitted light.